DC permanent magnet torque and armature current charateristics

[SMUG]

Does anybody know why DC Permanent magnet motors have constant torque and then drop down torque for high speeds.
Is it due to PWM??

On the other hand conventional motors just show a drop down torque/current defined by no-load and full load points.

 

[Skogsgurra]

The first thing that happens is that you get commutation problems as speed increases.

Then, you have more windage and other friction that reduces available torque on the output shaft.

There's also the inductance in the rotor winding that gets more and more pronounced when speed increases. Since armature current needs to change faster when speed goes up, there is a limit from where full torque isn't available any more.

And, but that is not for PM motors, the excitation is often reduced to run above base speed. That makes the torque constant Kt lower, which is self-evident.

What size, voltage, make, speed range are you having problems with?

Gunnar Englund

http://www.gke.org

--------------------------------------
100 % recycled posting: Electrons, ideas, finger-tips have been used over and over again...

 

[Clyde38]

They don't. A PM DC motor has torque proportional to the current applied. It has a generated voltage (back EMF) that will be closer to the applied voltage as the speed increases. This will limit the amount of current the motor can use for torque.

 

[SMUG]

It is a brushless motor with torque constant of 2Nm/amp and Kv about 182V/1000rpm.

Now I want to model the motor in simulink and I wonder conventional electrical/mechanical equations for a dc motor would not be applicable.

Clyde, You mentioned that increase in back emf will reduce the current flow which make sense but do we use PWM for voltage supply for brushless motors.

The other question that comes up is uptil speed the torque output remains constant ie what controls the knee point.

 

[benta]

"A PM DC motor has torque proportional to the current applied."
No! A PM DC motor draws current proportional to the torque required by the load.

Benta.

 

[Skogsgurra]

SMUG,

The way you put the question: "Does anybody know why DC Permanent magnet motors have constant torque and then drop down torque for high speeds" is misleading.

Of course, the back EMF reduces current. That is known by everyone and I do not understand how you can even start a simulation without including that property in your model.

Is that what you were asking about? Or did you want to know why the Kt falls back att higher speeds? Or why you are not allowed to load a DC motor fully at higher speeds? And, please, do tell what motor type you are asking about. You say PM DC motor, but ask about a BLDC motor.

We want to help, but we (me) don't want to look like complete fools when answering "fuzzy" questions.

Regarding the use of PWM in BLDC motors. It depends. Most small motors just switch from winding to winding. Larger motors may or may not use PWM to get a smoother operation.

Gunnar Englund

http://www.gke.org

--------------------------------------
100 % recycled posting: Electrons, ideas, finger-tips have been used over and over again...

 

[Clyde38]

"We want to help, but we (me) don't want to look like complete fools when answering "fuzzy" questions."

Oh, it doesn't seem to stop me...
See below...

"A PM DC motor has torque proportional to the current applied."
No! A PM DC motor draws current proportional to the torque required by the load.

Benta.

Thanks for the help Benta. I'd love to use the excuse that it was early........

Smug,
You might want to get a book called "Design of Brushless Permanent-Magnet Motors" by J. R. Hendershot and T.J.E Miller. I think you will enjoy.

 

[Skogsgurra]

I do not think that your answer was that bad, Clyde. If you apply a current to a DC motor - which is perfectly valid situation and happens in a lot of applications, like a center driven winder - then the torque developed is proportional to applied current.

Gunnar Englund

http://www.gke.org

--------------------------------------
100 % recycled posting: Electrons, ideas, finger-tips have been used over and over again...

 

[cswilson]

If you spin a motor with a fixed magnetic field, as is produced by the permanent magnets of the motors in question, it will produce a back EMF voltage on the armature terminals proportional to the velocity. The fundamental equation is:

E = Ke * w

where E is the back EMF voltage, w is the motor velocity, and Ke is the back-EMF constant. For brushless motors, the back EMF is an AC waveform, so E can be expressed as either a peak or RMS magnitude.

It does not matter how you spin the motor -- the back EMF voltage will be there. If you spin it mechanically, as with a steam turbine, the back EMF is pretty much the only contribution to terminal voltage. This is how we generate the vast majority of the world's electricity.

You can also spin the motor by applying current to the armature terminals. This will require an additional voltage for the I*R needed to force the current through the armature resistance. (We'll ignore L*di/dt effects for now.) The key is that this voltage is in addition to the back EMF voltage due to the motor speed.

SMUG -- I am virtually certain you are asking about torque LIMITS as a function of speed. So the question is what limits you at various speeds.

Most industrial servo motors designed to run off hundreds of volts (as yours are) are limited by current at low speeds. Too much current can (a) fry the winding insulation, (b) demagnetize the magnets, and/or (c) overheat the motor. (This further means that if you apply full supply voltage to a stopped or slowly moving motor, even very briefly, you will fry the motor somehow. Remember that these motors have different instantaneous current limits (for degmagnetizing and insulation breakdown) and continuous current limits (for overheating)

Since motor torque is proportional to current (you were correct, Clyde), when you are current limited, you are torque limited. This will apply up until the speed where the total terminal voltage V = Ke*w + Imax*R equals the supply voltage. Above this speed, you have less and less voltage "headroom" to apply current, so the amount of current you can apply, and therefore the amount of torque you can generate, falls off pretty much linearly as speed increases, up until the speed where back EMF equals supply voltage, at which point there is no capability to apply current and generate torque. This is the "no-load" speed.

It does not matter how you are modulating from the supply voltage -- PWM, linear modulation, or other -- this argument applies.

Note that many motors designed to run from very low voltages, say 12-48V, are voltage limited over the entire speed range.

Looking back on your questions, you may have some confusion between torque limits for a motor and the torque created for a given voltage and current. The torque will always be proportional to current, and the voltage will be the sum of the back EMF plus the electrical drops, mainly IR. These relationships will apply anywhere within the outer envelope of performance described by the curve you mention.

Curt Wilson
Delta Tau Data Systems